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package org.openjdk.jmh.samples;


import org.openjdk.jmh.annotations.*;

import org.openjdk.jmh.runner.Runner;

import org.openjdk.jmh.runner.RunnerException;

import org.openjdk.jmh.runner.options.Options;

import org.openjdk.jmh.runner.options.OptionsBuilder;


import java.util.concurrent.TimeUnit;


@State(Scope.Thread)

@BenchmarkMode(Mode.AverageTime)

@OutputTimeUnit(TimeUnit.NANOSECONDS)

public class JMHSample_08_DeadCode {


 /*

  * The downfall of many benchmarks is Dead-Code Elimination (DCE): compilers

  * are smart enough to deduce some computations are redundant and eliminate

  * them completely. If the eliminated part was our benchmarked code, we are

  * in trouble.

  *

  * Fortunately, JMH provides the essential infrastructure to fight this

  * where appropriate: returning the result of the computation will ask JMH

  * to deal with the result to limit dead-code elimination (returned results

  * are implicitly consumed by Blackholes, see JMHSample_09_Blackholes).

  */


 private double x = Math.PI;


 @Benchmark

 public void baseline() {

  // do nothing, this is a baseline

 }


 @Benchmark

 public void measureWrong() {

  // This is wrong: result is not used and the entire computation is optimized away.

  Math.log(x);

 }


 @Benchmark

 public double measureRight() {

  // This is correct: the result is being used.

  return Math.log(x);

 }


 /*

  * ============================== HOW TO RUN THIS TEST: ====================================

  *

  * You can see the unrealistically fast calculation in with measureWrong(),

  * while realistic measurement with measureRight().

  *

  * You can run this test:

  *

  * a) Via the command line:

  *    $ mvn clean install

  *    $ java -jar target/benchmarks.jar JMHSample_08 -f 1

  *    (we requested single fork; there are also other options, see -h)

  *

  * b) Via the Java API:

  *    (see the JMH homepage for possible caveats when running from IDE:

  *      http://openjdk.java.net/projects/code-tools/jmh/)

  */


 public static void main(String[] args) throws RunnerException {

  Options opt = new OptionsBuilder()

          .include(JMHSample_08_DeadCode.class.getSimpleName())

          .forks(1)

          .build();


  new Runner(opt).run();

 }
// 本例主要介绍了一个知识点：Dead-Code Elimination (DCE) ，即死码消除，文档上说编译器非常聪明，有的代码没啥用，就在编译器被消除了，但这给我做基准测试带了一些麻烦，比如上面的代码中，baseline 和 measureWrong 有着相同的性能，因为编译器觉得 measureWrong这段代码执行后没有任何影响，为了效率，就直接消除掉这段代码，但是如果加上return语句，就不会在编译期被去掉，这是我们在写基准测试时需要注意的点。


}